Evaluation of hexagonal chiral structure for morphing airfoil concept

Abstract

In this paper a concept of hexagonal chiral honeycomb is proposed as a truss-like internal structure for adaptive wing box configurations. In contrast with classical centresymmetric cellular structures like rectangular or hexagonal grids, the proposed honeycomb did not present inversion symmetry, and featured an in-plane negative Poisson's ratio behaviour. The cellular structure considered exhibited this Poisson's ratio behaviour under a large range of strain. A set of numerical (finite element, FE) simulations have been carried out in order to correct the initial theoretical predictions to take into account axial, shear and elastic deformations of all elements composing the unit cell when subjected to uniaxial loading. The homogenized linear elastic mechanical properties were then introduced in an FE wing box model of a racecar wing coupled to a panel code to simulate unidirectional static fluid -structure coupling between the wing box and the flow surrounding the airfoil. The cellular solid proposed as the internal layout of the wing box allowed conforming deformations with the external flow, giving a variation of the camber line and trailing edge displacement, and acting as an aileron.

Numerical and experimental analysis of the static compliance of chiral truss-core airfoils

“…Initial investigations on the application of the chiral geometry to morphing can be found in [Bornengo et al 2005], where the performance of a conformable race car wing is analyzed through a numerical model. In [Bornengo et al 2005], the airfoil core is modeled as a homogeneous material with the mechanical properties of a homogenized chiral assembly. This implicitly assumes that the unit cell size is much smaller than the dimensions of the wing.…”

“…Subsequent studies have instead considered configurations where the cell has dimensions of the order of those of the structure (Figure 3). Both in [Bornengo et al 2005] and in , an Eppler 420 profile is considered. Such a highly cambered airfoil is chosen to demonstrate the compliance of the assembly, as the deformations that are sought involve decambering effects.…”

“…A similar, remarkable solution for camber variation is the finger concept [Monner et al 2000], where the airfoil features a flexible rib composed of plate-like elements connected through revolute joints. Tension-torsion coupling has been employed as an effective means to actively control camber in helicopter blades [Büter et al 2001], while other design solutions have considered inflatable airfoil structures [Cadogan et al 2004], Keywords: morphing airfoils, aeroelastic tailoring, chiral topology, truss-core airfoils. The authors wish to thank the Army Research Office (ARO) for the support provided for this work under the grant 45518-EG.…”

Dynamic Response of Chiral Truss-core Assemblies

“…Much interest has recently centered on cellular assemblies featuring a negative Poisson's ratio behavior, also known as 'auxetic' (Lakes, 1991). Materials having auxetic characteristics include special subsets of foams (Lakes, 1987), long-fiber composites (Alderson et al, 1997), microporous polymers (Lakes, 1991), as well as honeycombs (Scarpa and Tomlinson, 2000;Bornengo et al, 2005). The mechanical properties of these alternatives to solid components have thoroughly been investigated by (Zang and Ashby, 1991;Gibson et al, 1997;Evans et al, 2001;Wicks and Hutchinson, 2001) among others.…”

Vibroacoustics and wave propagation of novel chiral honeycombs

“…When subjected to in-plane loading, the cylinders would rotate, leading to winding/unwinding of the ligaments, and therefore providing the NPR effect. The Poisson's ratio value of -1, matching the one of the elasticity limit, makes chiral structures extremely interesting, due to their enhanced in-plane torsional stiffness and deformation characteristics, for applications not only limited to classical sandwich structures, but also wing box sections with morphing characteristics [3,4,5]. The first chiral topology was proposed at molecular level by Wojciechowski [6], and as structural honeycomb component by Prall and Lakes [7].…”